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Marsupials have acquired one of the most unique adaptations in the entire animal kingdom. These devoted mothers always keep their offspring with them, hidden in their abdominal pouch. They really are not like other animals on the planet.
Some of the most famous marsupials include wombats, wallabies, koalas, bandicoots, possums, kangaroos (as well as tree kangaroos), and Tasmanian devils. It is estimated that 70% of the more than 250 existing species are found exclusively in Australia. The remaining 30% are located in South America and Central America. Only one species (Virginia opossum) is found north of Mexico.
Marsupials are one of the three main groups of mammals, along with placental mammals and monotremes (egg-laying mammals). Genetic evidence suggests that they separated from placental mammals in the Cretaceous period, about 125 million years ago, and evolved independently from there. Despite their close ties to Australia, marsupials actually originated in what is now North America when it was still part of a larger supercontinent called Laurasia. About 70 million years ago, the marsupials reached South America and Australia, which at the time were almost connected to each other across Antarctica.
When Australia separated from other continents a short time later, wildlife developed there separately from everything else on the rest of the planet. Without competition from placental mammals (which may have become extinct early or simply never appeared), marsupials thrived in an isolated Australian ecosystem. They may have also supplanted the existing monotreme populations, competing with them for resources.
Marsupials differ from placental mammals in several important ways. The ability to feed babies in pouches is probably the most significant anatomical difference between marsupials and placental mammals. They also have a simpler social structure and less varied vocalizations. But in general, placental mammals and marsupials have more similarities than differences. This article will explore eight main reasons why marsupials are classified as mammals.
Marsupials have mammary glands.
The ability to produce milk through the mammary glands is a hallmark of all mammals. Even mammals that lay eggs, such as the platypus and echidna, will feed their young with milk. In marsupials, milk is produced from the nipples located inside or sometimes above the pouch.
Although they can produce milk, the actual life cycle of marsupials differs significantly from placental mammals. Newborn offspring (also known as joey) are born at a very early stage of development, when they are still as small as beans and hairless. After about a few weeks, Joey will crawl out of the birth canal into the mother’s pouch and remain attached to the nipple until he is ready to start learning how to survive on his own. This roughly corresponds to the end of intrauterine development of placental mammals.
Although some marsupials have a temporary placenta, the development of marsupials is still very different from that of placental mammals, despite the presence of mammary glands. Even after leaving the bag, Joey sometimes returns there in search of safety and shelter, until he becomes completely independent.
Some scientists have actually proposed a theory that prolonged nipple sucking limits the marsupial’s ability to evolve into different forms. Because they need to suckle at such an early stage in their development, marsupials develop jaws even before they have a fully functioning brain. This means (in scientific jargon) there is strong selective pressure to maintain the current jaw shape. He cannot easily change and develop in response to new conditions. This may be one of the reasons why marsupials are relatively limited in shape, while placental mammals have evolved incredibly diverse forms, from whales to dogs.
Marsupials have fur
Mammals are one of the few animals in which thick fur grows mainly for the purpose of isolation; hair color seems to directly affect the amount of heat retained in the body. Fur also serves many other purposes, including sensory feedback, camouflage, and soft skin protection.
Wombats and Tasmanian devils also have fur that appears to glow in ultraviolet light, but it’s unclear what purpose this serves. Some species of marsupials have such thick and luxurious fur that they are hunted almost to extinction to make clothes and other clothing.
Warm-blooded marsupials
Warm-bloodedness (also known by the scientific term endothermia) refers to the ability of an animal to maintain a warm and comfortable body temperature even when the ambient temperature drops. To maintain this temperature, mammals tend to have a much higher metabolic rate and therefore need more energy to fuel it. Endothermia is very common in mammals and birds, but some fish are also considered warm-blooded.
Marsupials have four-chambered hearts.
The four-chambered heart is one of the main innovations of both birds and mammals. While reptiles only have a three-chambered heart and other vertebrates only have two, mammals have developed a more complex circulatory system that allows the heart to more efficiently sort the oxygenated blood leaving the lungs from the depleted blood entering the lungs. This ensures that the animal’s cells receive only the most oxygen-rich blood, so it can expend more energy over longer periods of time.
In marsupials, the diaphragms are strong.
The diaphragm is the chest muscle that expands and contracts the lungs. It also separates the heart and lungs from the abdomen. Mammals probably have the most developed diaphragm in the entire animal kingdom (although birds also have a strong diaphragm). Combined with a four-chambered heart and a fast metabolism, this allows them to use energy more efficiently than other animals (and therefore utilize a wider range of ecosystems).
The red blood cells of marsupials do not have a nucleus.
Mammals are the only class of vertebrates whose blood cells do not contain any nuclei. The nucleus is the part of a cell that contains genetic information. Without a nucleus, red blood cells can carry even more oxygen-rich hemoglobin throughout the body, thereby fueling the high-energy activity of mammals.
Marsupials have a mammalian skeleton
Mammals differ from other animal species by the evolution of the skull. While reptiles have multiple parts of the jaw, mammals have a solid bone in the lower jaw, known as tartar, that attaches directly to the rest of the skull. A single bone allows mammals to cut and chew meat or grind plant matter with incredible power.
Scientists now believe that at some point in early mammalian evolution, the bones of the lower jaw of reptiles moved further back into the head, forming part of the auditory system that transmits sound to the innermost ear.
Marsupials have complex brains
The brain of marsupials is not as complex as that of placental mammals. They lack the part of the brain that connects the two halves together. Their brains are also smaller compared to their overall body size. However, the relatively complex structure of the brain, at least compared to many other animals, does allow them to exhibit complex behaviors.
Next: what do sea urchins eat?
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